In tires for rough roads or heavy loads, for the purpose of enhancing abrasion resistance, low structure carbon black has hitherto been widely used. However, in order to prevent occurrence of the matter that rubber is soft so that it is easily deformed, the carbon black is frequently highly filled. For that reason, there was involved a problem that not only dispersion of carbon black in the rubber is deteriorated so that sufficient abrasion resistance is not obtained, but also increased heat generation is caused.
In order to improve processability, reinforcing properties and abrasion resistance, there is known a production method of a natural rubber master batch including an amide linkage cleavage step of cleaving amide linkages in a natural rubber latex and a step of mixing the latex after the subject amide linkage cleavage step and an aqueous slurry of an inorganic filler such as carbon black, silica, alumina, calcium carbonate, etc. dispersed in water (see, for example, Patent Document 1).
However, Patent Document 1 (JP-A-2004-99625) does not disclose means for enhancing dispersibility of carbon black. Accordingly, in tires using the natural rubber master batch obtained by the production method of Patent Document 1, it is unclear whether or more low heat build-up is obtainable.